At present, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, and life span is short. However, in recent years, a high luminescence LED emitting light of near ultraviolet/ultraviolet to blue color has been developed in sequence, and the white LED illumination for the practical application of the next generation has been actively studied and developed, in which the white light is prepared by mixing the light of the near ultraviolet/ultraviolet to blue color generated from the LED and the light generated from the phosphor having an excitation band in a wavelength region thereof. When the white LED illumination is put to practical use, since efficiency of converting electric energy into light is improved, less heat is generated and it is constituted of the LED and a phosphor, the white LED has advantages of good life span without burn-out of a filament as is seen in a conventional incandescent bulb and the harmful substance such as mercury is not contained, and miniaturization of the illumination device is realized, thus realizing an ideal illumination device.
A white LED illumination system creating white light by combining the high luminance LED and the phosphor is called one chip system. This one chip system has an advantage that it has excellent color rendering properties and can be manufactured at a low cost, compared to a multi-chip type system which creates white color by using three primary color LEDs such as high luminance red LED, green LED, and blue LED, and is focused as an illumination of the next generation.
As the white LED illumination of the one chip system, there is one that combines the high luminance blue LED and the phosphor emitting yellow light under an excitation of blue emitting light generated from the LED, and for example, which is obtained by combining the high luminance blue LED, in which an InGaN material is used, and garnet yellow phosphors such as (Y, Gd)3(Al, Ga)5O12:Ce(YAG:Ce), Tb3Al5O12:Ce, and Ca3Sc2Si3O12:Ce. In the white LED illumination, white color is obtained by using a complementary relation between blue emission of the LED and yellow emission of the phosphor. However, the problem of the white LED illumination is that the emission on the long-wavelength side of visible light range, specifically the emission of red color component is insufficient although the white LED illumination has a high luminance, thereby deteriorating in the color rendering properties which are essential factors of illumination. However, in recent years, the phosphor having an excitation spectrum with a peak in the wavelength range from yellow color to red color, and also having a good excitation band in a range from near ultraviolet/ultraviolet to blue color, has been developed in sequence. Then, by adding such a phosphor, the color rendering properties are improved. As such phosphors containing nitrogen, Ca2Si5N8:Eu, Sr2Si5N8:Eu, Ba2Si5N8:Eu, Cax(Al, Si)12(O, N)16:Eu (0<x≦1.5), CaAl2Si4N8:Eu, CaSiN2:Eu, CaAlSiN3:Eu are typically given as examples.
However, problems are involved in the white LED illumination, in which the high luminance blue LED and the garnet yellow phosphor are combined, such that the garnet yellow phosphor does not have a flat broad excitation band near the excitation wavelength of 460 nm, there are variances in the emission intensity and the peak wavelength of the high luminance blue LED, and the emission intensity of transmitted blue light depending on a film thickness is changed when the phosphor is applied on the LED. And these facts cause a lost balance of the emission intensity of blue color and yellow color, thereby changing a color tone of the white light.
In order to solve the above-described problem, at present, a white LED illumination system is actively studied. In such a white LED illumination system, white color is obtained by using the light in a mixed state of the near ultraviolet/ultraviolet emitting LED and the red (R) color emitting phosphor, the green (G) color emitting phosphor, and the blue (B) color emitting phosphor excited by the light of the near ultraviolet/ultraviolet light generated from the LED. This system has an advantage that an arbitrary emission color can be obtained in addition to white light by the combination of the R, G, B, and the mixing ratio, white emission is obtained not by the complementary relation of light but by the mixing state of light, and by using the R, G, B and other phosphors having broad emission spectra, the emission spectrum which is approximated the spectrum of the sun-light is obtained compared to the white LED obtained by combining the high luminance blue LED and the garnet yellow phosphor, and therefore the color rendering properties are possibly improved. Further, even when there is a variance in the emission intensity and the peak wavelength of the excitation light_as is seen in the high luminance blue LED, a phenomenon of changing the color tone of the white light does not occur, because the near ultraviolet/ultraviolet light is not used in the mixing state of the light. Therefore the white LED illumination with excellent color rendering properties and small in variance of the color tone can be prepared.
Then, as the phosphor used for such an application, examples are given such as Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu for the red phosphor, ZnS:Cu, Al, CaGa2S4:Eu, SrGa2S4:Eu, BaGa2S4:Eu, SrAl2O4:Eu, BAM:Eu, Mn, Ba2SiO4:Eu for the green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu for the blue phosphor. In the phosphor containing nitrogen as described above, Ca2Si5N8:Eu, Sr2Si5N8:Eu, Ba2Si5N8:Eu, Cax(Al, Si)12(O, N)16:Eu(0<x≦1.5), CaAl2Si4N8:Eu, CaSiN2:Eu, CaAlSiN3:Eu and so forth have high efficient excitation band even in the near ultraviolet/ultraviolet and emission spectra with broad peaks, and therefore have improved luminance and color rendering properties even in the white LED illumination obtained by combining the near ultraviolet/ultraviolet LED and the R, G, B and other phosphor. However, there is no high efficient and high luminance R, G, B and other phosphor such as the YAG:Ce phosphor used in combination with the high luminance blue LED and the garnet yellow phosphor, and therefore a satisfactory white LED illumination can not be obtained.
Therefore, in regards to the phosphor of each color, a new phosphor having an excellent emission characteristic has been developed, and in regards to the blue phosphor also, a new blue phosphor surpassing the present BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu has been actively developed. In recent years, nitrogen-containing La1−xSi3N5:Cex (for example, see patent document 1) is reported.
(Patent document 1) Japanese Patent Laid-Open No.2003-96446
However, although the nitrogen-containing phosphor of the aforementioned patent document 1 has the emission spectrum with a broad peak, the emission intensity under the excitation of the excitation light of near ultraviolet/ultraviolet does not meet a satisfactory level, and therefore a sufficient emission intensity and luminance can not be obtained. Thus, the phosphor of the patent document 1 is inadequate to be used in the light emitting device.